[slack-22.0]: backport 18967: connpool: fix connection leak during idle connection reopen

go/pools/smartconnpool/pool.go

@@ -460,9 +460,13 @@ func (pool *ConnPool[C]) pop(stack *connStack[C]) *Pooled[C] {
 	// to expire this connection (even if it's still visible to them), so it's
 	// safe to return it
 	for conn, ok := stack.Pop(); ok; conn, ok = stack.Pop() {
-		if conn.timeUsed.borrow() {
-			return conn
+		if !conn.timeUsed.borrow() {
+			// Ignore the connection that couldn't be borrowed;
+			// it's being closed by the idle worker and replaced by a new connection.
+			continue
 		}
+
+		return conn
 	}
 	return nil
 }
@@ -787,11 +791,23 @@ func (pool *ConnPool[C]) closeIdleResources(now time.Time) {
 		for conn := s.Peek(); conn != nil; conn = conn.next.Load() {
 			if conn.timeUsed.expired(mono, timeout) {
 				pool.Metrics.idleClosed.Add(1)
+
 				conn.Close()
+				pool.closedConn()
+
 				// Using context.Background() is fine since MySQL connection already enforces
 				// a connect timeout via the `db_connect_timeout_ms` config param.
-				if err := pool.connReopen(context.Background(), conn, mono); err != nil {
-					pool.closedConn()
+				c, err := pool.getNew(context.Background())
+				if err != nil {
+					// If we couldn't open a new connection, just continue
+					continue
+				}
+
+				// opening a new connection might have raced with other goroutines,
+				// so it's possible that we got back `nil` here
+				if c != nil {
+					// Return the new connection to the pool
+					pool.tryReturnConn(c)
 				}
 			}
 		}

go/pools/smartconnpool/pool_test.go

@@ -1322,3 +1322,88 @@ func TestCloseDuringWaitForConn(t *testing.T) {
 		require.EqualValues(t, 0, state.open.Load())
 	}
 }
+
+// TestIdleTimeoutConnectionLeak checks for leaked connections after idle timeout
+func TestIdleTimeoutConnectionLeak(t *testing.T) {
+	var state TestState
+
+	// Slow connection creation to ensure idle timeout happens during reopening
+	state.chaos.delayConnect = 300 * time.Millisecond
+
+	p := NewPool(&Config[*TestConn]{
+		Capacity:    2,
+		IdleTimeout: 50 * time.Millisecond,
+		LogWait:     state.LogWait,
+	}).Open(newConnector(&state), nil)
+
+	getCtx, cancel := context.WithTimeout(t.Context(), 500*time.Millisecond)
+	defer cancel()
+
+	// Get and return two connections
+	conn1, err := p.Get(getCtx, nil)
+	require.NoError(t, err)
+
+	conn2, err := p.Get(getCtx, nil)
+	require.NoError(t, err)
+
+	p.put(conn1)
+	p.put(conn2)
+
+	// At this point: Active=2, InUse=0, Available=2
+	require.EqualValues(t, 2, p.Active())
+	require.EqualValues(t, 0, p.InUse())
+	require.EqualValues(t, 2, p.Available())
+
+	// Wait for idle timeout to kick in and start expiring connections
+	require.EventuallyWithT(t, func(c *assert.CollectT) {
+		// Check the actual number of currently open connections
+		assert.Equal(c, int64(2), state.open.Load())
+		// Check the total number of closed connections
+		assert.Equal(c, int64(1), state.close.Load())
+	}, 100*time.Millisecond, 10*time.Millisecond)
+
+	// At this point, the idle timeout worker has expired the connections
+	// and is trying to reopen them (which takes 300ms due to delayConnect)
+
+	// Try to get connections while they're being reopened
+	// This should trigger the bug where connections get discarded
+	for i := 0; i < 2; i++ {
+		getCtx, cancel := context.WithTimeout(t.Context(), 50*time.Millisecond)
+		defer cancel()
+
+		conn, err := p.Get(getCtx, nil)
+		require.NoError(t, err)
+
+		p.put(conn)
+	}
+
+	// Wait a moment for all reopening to complete
+	require.EventuallyWithT(t, func(c *assert.CollectT) {
+		// Check the actual number of currently open connections
+		require.Equal(c, int64(2), state.open.Load())
+		// Check the total number of closed connections
+		require.Equal(c, int64(2), state.close.Load())
+	}, 400*time.Millisecond, 10*time.Millisecond)
+
+	// Check the pool state
+	assert.Equal(t, int64(2), p.Active())
+	assert.Equal(t, int64(0), p.InUse())
+	assert.Equal(t, int64(2), p.Available())
+	assert.Equal(t, int64(2), p.Metrics.IdleClosed())
+
+	// Try to close the pool - if there are leaked connections, this will timeout
+	closeCtx, cancel := context.WithTimeout(t.Context(), 500*time.Millisecond)
+	defer cancel()
+
+	err = p.CloseWithContext(closeCtx)
+	require.NoError(t, err)
+
+	// Pool should be completely closed now
+	assert.Equal(t, int64(0), p.Active())
+	assert.Equal(t, int64(0), p.InUse())
+	assert.Equal(t, int64(0), p.Available())
+	assert.Equal(t, int64(2), p.Metrics.IdleClosed())
+
+	assert.Equal(t, int64(0), state.open.Load())
+	assert.Equal(t, int64(4), state.close.Load())
+}